Shuttle-type printers and methods for operating same

Abstract: Print quality in shuttle-type printers is monitored by providing a printhead (40) with an optical sensor (50) which optically senses a recording media (14) to establish a background reflective level, moving the printhead to a location of a selected second optical density to establish a foreground reflective level, printing images on the media, and optically sensing the images and comparing with the background and foreground levels to detect any changes in reflectance levels of the sensed images indicating changes in print quality.

Description: Technical Field This invention relates to shuttle-type printers and methods for operating them. Background of the Invention Shuttle-type printers are a class of printers having a movable shuttle or carriage that traverses back and forth across a printing surface. A printhead is mounted on the shuttle and synchronized with shuttle movement to print desired images. The shuttle class of printers includes both impact printers, such as dot matrix and daisy-wheel printers, and non-impact printers, such as ink-jet printers. A shuttle drive mechanism maneuvers the shuttle over the printing surface. The shuttle drive mechanism typically consists of a motor, and a belt and pulley assembly which operably couples the shuttle to the motor. Common motors used in such mechanisms include a DC motor which changes speed and direction in relation to the level and polarity of DC voltage applied thereto, and a stepper motor which changes speed and direction in response to intermittent pulses. The stepper motor is less effective at providing precise position control as compared to the DC motor plus shaft encoder; but, the stepper motor is advantageously less expensive than the DC motor and encoder. One problem that plagues shuttle-type printers is the inherent lack of precise positional control due to mechanical tolerances of the shuttle drive mechanism. the motor and drive belt assembly possess manufacturing variances that induce slight, but acceptable, errors in the shuttle positioning process. These errors are manifest in assembled printers and vary from printer to printer. Accordingly, it would be advantageous to identify the inherent mechanical errors within an assembled printer and compensate for them. A solution for determining absolute carriage position relative to a platen is described and claimed in European patent Application No. 94307814.7 (EP-A-0650844), from which this present Application is divided out. Another problem associated with printers concerns maintaining consistent print quality. Generally, print quality tends to deteriorate over time. This deterioration may be the result of mechanical wear or other factors such change in ink drop-volume (for ink-jet printers) or variations in pin impact (for dot matrix printers). while degradation in print quality is traditionally detected by the user, it would be desirable to provide an automated approach to monitoring print quality. The present invention, as specified in the claim hereinafter, provides a method of operating a shuttle-type printer that monitors print quality. Brief Description of the Drawings Preferred embodiments of the invention are described below with reference to the following accompanying drawing depicting examples embodying the best mode for practising the invention. Fig. 1 is a diagrammatic illustration of a printing system for a shuttle-type printer according to this invention. Detailed Description of the Preferred Embodiments Fig. 1 shows a printing system 10 of a shuttle-type printer. System 10 includes a platen 12, a shuttle assembly 20, a printhead 40, an optical sensor 50, and a control subsystem 60. Platen 12 is preferably stationary and supports a recording media 14 during printing. Recording media 14 has an upper edge 15, a first side edge 16, and a second side edge 18. Media 14 may be a continuous form or individual sheet stock, and it can consist of paper, adhesive-backed labels, or other types of printable matter. A media feed mechanism (not shown), such as friction rollers or a tractor feed system, is used to drive the media through the printer along a media feed path. The media feed path is represented by dashed boundary lines 19 and has a width effective to coincide with a first portion of platen 12 while leaving exposed a second portion of the platen. More specifically, platen 12 has a center region 17 that defines media feed path 19 and two opposing end regions 21, 23 that extend beyond the media feed path. Shuttle assembly 20 includes a carriage 22 slidably mounted on a fixed, elongated rod 24 to move bidirectionally across the platen 12. Carriage 22 preferably maneuvers over the full width of the platen to be positionable over the media feed path 19 at the platen center region 17 and over the two opposing end regions 21, 23 outside of media feed path 19. Carriage 22 has a nose section 25 that is adjacent to, but spaced from, the platen 12 to permit passage of the recording media 14 therebetween. Shuttle assembly 20 further includes a drive subassembly 26 that is mechanically coupled to drive carriage 22 back and forth along rod 24. Drive subassembly 26 includes a wire or belt 28 attached to carriage 22 and wound around opposing pulleys 30, and a motor 32 connected to power one of the pulleys. Preferably, motor 32 is a stepper motor, but a DC motor can also be used. A rotary encoder 34 is coupled to the motor drive shaft to monitor incremental shaft rotation. This incremental count provides feedback data for use in positioning and controlling the carriage. The shuttle assembly 20 is illustrated in one typical form for explanation purposes and its construction is well known in the art. However, other types of shuttle assembly configurations may be employed in this invention. Printhead 40 is mounted on nose section 25 of carriage 22 in juxtaposition with platen 12. Printhead 40 is diagrammatically represented as a block on nose section 25 of carriage 22 and can be embodied as an ink-jet printhead, a dot matrix printhead, a daisy-wheel, or any other type of printhead carried on a shuttle. An optical sensor 50 is also mounted on carriage 22 to be positionable above platen 12 and/or media 14. Optical sensor 50 includes a light source (e.g., photoemitter, LED, laser diode, super luminescent diode, fiber optic source) oriented to emit a light beam toward platen 12 and a light sensitive detector (e.g., photodetector, charged couple device, photodiode) aligned to detect light reflected from the platen or media. Optical sensor 50 is preferably mounted adjacent to, and in substantial alignment with, the printhead 40 to monitor lines of text or other images that have already been printed. The control subsystem 60 of printing system 10 consists of various components used to monitor and control operation of the printing system. It includes a printhead controller 62, an optical sensor controller 64, a carriage controller 66, a memory 68, and a processor 69. These components are illustrated in block form for clarity of discussion. Printhead controller 62 is electrically coupled to printhead 40 to manage the tasks associated with transforming digital data downloaded to the printer into desired patterns to be applied on the recording media. Optical sensor controller 64 is electrically coupled to monitor signals generated by optical sensor 50. Carriage controller 66 is configured to manage motor 32 and receive incremental motion feedback from rotary encoder 34 to controllably position carriage 22 at selected locations relative to platen 12 or media 14. Memory 68 is preferably a non-volatile, randomly accessible memory which stores position-related information. In practice, control subsystem 60 is embodied as one or more microprocessors, microcontrollers, ASICs, or other circuitry and logic. Printing system 10 also has at least one optically responsive platen demarcation 70 provided at one end 21 of platen 12. Preferably, a platen demarcation is provided at each of the two opposing end regions 21 and 23 outside of media feed path 19, as shown by demarcations 70 and 72, respectively. In this manner, when media 14 is fed through printing system 10 between carriage 22 and platen 12, the demarcations 70 and 72 remain exposed beside the media. The demarcations possess a distinctly different optical density as compared to that of the platen to induce a detectable change in signal output when the optical sensor 50 passes over the demarcation. The demarcations are embodied as apertures formed in the platen. the demarcations 70, 72 are used in conjunction with optical sensor 50 to enable measurement of absolute carriage position relative to platen 12, as is described and claimed in EP-A-0650844. Print Quality The method according to this invention concerns a simple, low cost approach to monitoring print quality. Once media 14 is fed into the printing system, optical sensor 50 takes a sample reading of the media to establish a background reflectance level. This level is stored in memory 68. The carriage 22 is then moved to a location having a marking of a selected optical density different than that of the media. By way of example only, the marking can be permanently provided on the platen or alternatively, preprinted on the recording media or deposited thereon by the printhead 40. The optical sensor 50 takes another sample reading of the marking to establish a foreground reflectance level different than the background reflectance level. The foreground reflectance level is also stored in memory 68. The printer is then operated in its normal printing mode to print images on the recording media 14. The optical sensor 50 routinely monitors the printed images and compares the sensed images with the background and foreground reflectance levels stored in memory 68 to detect any changes in reflectance of the sensed images. Over time, the print quality of the printed images degrades (due to shortage of ink, change in pin impact strength, etc.), causing an identifiable change in reflectance. When the monitored reflectance changes relative to the preferred stored levels, the control subsystem 60 warns the user that the print quality may be deteriorating.

Claims: A method for operating a shuttle-type printer, the shuttle-type printer having a platen (12), a carriage (22) which moves bidirectionally across the platen, and a printhead (40) and an optical sensor (50) mounted on the carriage, the method comprising the following steps: feeding a recording media (14) of a first optical density between the platen (12) and carriage (22) along a media path (19); optically sensing the recording media (14) to establish a background reflectance level; moving the carriage (22) to a location having a marking of a selected second optical density; optically sensing the marking to establish a foreground reflectance level different than the background reflectance level; printing images on the recording media (14); optically sensing the images printed on the recording media; and comparing the sensed images with the background and foreground reflectance levels to detect changes in reflectance of the sensed images, the reflectance changes indicating changes in print quality.

Inventors: KHORMAEE IZADPOUR, KHORMAEE, IZADPOUR
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1994
Main classification: B41J19
IPC codes: B41J25, B41J11, B41J29